Currently, transfer and access technologies in mobile communication system are rapidly improving such as start of service of the international mobile telecommunication 2000 (IMT-2000). Moreover, technologies such as a high speed down-link packet access (HSDPA) are standardized and data transfer of around 10 Mbps at maximum is expected to be in practical use.
Meanwhile, standardization to realize broadband wireless internet access aiming at 10 to 100 Mbps transfer rate is also being promoted and various techniques have been proposed.
A prerequisite for realizing wireless communication with high speed transfer rate is raising spectrum efficiency. Because there is a proportional relation between the transfer rate and bandwidth to be used, expansion of a frequency bandwidth to be used is a simple solution for raising the transfer rate. However, available frequency bandwidth is tight and it is unlikely that sufficient bandwidth is allocated if a new wireless communication system is constructed. Therefore, it is required to raise spectrum efficiency.
Moreover, another prerequisite is to provide a service in a private area (independent cell) such as wireless LAN seamlessly while realizing service in a communication area configured by a cell such as a mobile phone.
There is a technique called one cell repeat orthogonal frequency division multiplexing/frequency division multiple access (hereinafter referred to as OFDMA) which has a possibility to solve all of the above. This is a technique for carrying out communication in a communication area including a cell by use of the same frequency. Modulation method thereof when communicating is OFDM, while time division multiple access (TDMA) or frequency division multiple access (FDMA) is used as an access method. Needless to say, this is a communication method which enables faster data communication in an independent cell with a wireless interface commonly used with the cell area.
The OFDM and FDMA, which are elemental technology for the OFDMA, will be explained below. The OFDM is a method used for IEEE802. 11a which is a wireless system with 5 GHz band or for a digital terrestrial broadcasting. The OFDM is a method by which several tens to thousands of carriers are aligned with a frequency interval, which is theoretically minimum to prevent interference, for simultaneous communication. Normally, the carriers are called sub-carriers in the OFDM and each sub-carrier is digital modulated by phase shift keying (PSK), quadrature amplitude modulation (QAM), or the like to carry out communication. Further, this is regarded as a modulation method strong against frequency selective fading when combined with an error correction method.
The FDMA is a method which accesses by dividing frequency when receiving/transmitting data. Normally in a communication system using the FDMA as an access method, frequency is divided into several bandwidth to divide frequency bandwidth for carrying out communication so that a terminal to be accessed is distinguished. Usually, a protection bandwidth called a guard band is prepared between frequency bandwidths thus divided. However, in the OFDMA, the guard band is not used to prevent damages to spectrum efficiency, or if used, it is a very narrow one with a bandwidth of some sub-carriers.
Moreover, as a method to increase communication efficiency in a multi-channel system such as the OFDMA, there is a method that causes channel quality information (CQI) of each channel to be reported from each terminal to a base station, the base station allocates the best channel to each terminal, and the best modulation method is selected among them so that a user diversity effect can be obtained. This is because in an environment where multipath or fading exists, frequency with better communication quality changes one right after the other and therefore allocating a channel having good characteristics at each time enables the terminal to obtain better communication quality than when a channel is allocated in a fixed manner.
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[Patent Document 2] Published Japanese translation of a PCT application No. 2004-510358
[Patent Document 3] Japanese Unexamined Patent Publication No. 2004-179853
[Patent Document 4] Japanese Unexamined Patent Publication No. 2004-253914
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[Patent Document 6] Japanese Unexamined Patent Publication No. 2005-86818